1. Technical Field
This invention concerns the field of filtration and more specifically, rotary filtration devices.
2. Background Art
Filtration devices are used to separate one or more components of a fluid from other components. Common processes carried out in such devices include classic filtration, microfiltration, ultrafiltration, reverse osmosis, dialysis, electrodialysis, pervaporation, water splitting, sieving, affinity separation, affinity purification, affinity sorption, chromatography, gel filtration, and bacteriological filtration. As used herein, the term "filtration" includes all of those separation processes as well as any other processes using a filter that separate one or more components of a fluid from the other components of the fluid.
Filtration processes make use of the greater filter permeability of some fluid components than others. As used herein, the term "filter" includes any article made of any material that allows one or more components of a fluid to pass through it to separate those components from other components of the fluid. Thus, the term "filter" includes metallic and polymeric cloth filters, semipermeable membranes and inorganic sieve materials (e.g., zeolites, ceramics). A filter may have any shape or form, for example, woven or non-woven fabrics, fibers, membranes, sieves, sheets, films, and combinations thereof.
The components of the feed fluid that pass through the filter comprise the "permeate" and those that do not pass (i.e., are rejected by the filter or are held by the filter) comprise the "retentate." The valuable fraction from the filtration process may be the retentate or the permeate or in some cases both may be valuable.
A common technical problem in all filtration devices is blinding or clogging of the filter. Permeate passing through the filter from the feed fluid layer adjacent to the feed side of the filter leaves a layer adjacent to or on that side of the filter having a different composition than that of the bulk feed fluid. This material may bind to the filter and clog its pores (that is, foul the filter) or remain as a stagnant boundary layer, either of which hinders transport of the feed fluid components trying to pass through the filter to the permeate product side of the filter. In other words, mass transport per unit area through the filter per unit time (i.e., flux) is reduced and the inherent sieving capability of the filter is adversely affected.
Generally, fouling of the filter is chemical in nature, involving chemisorption of substances in the feed fluid onto the filter's internal (pore) and external surface area. Unless the chemical properties of the filter surface are altered to prevent or reduce adsorption, frequent and costly filter replacement or cleaning operations are necessary.
One of the most common causes of fouling arises from the low surface energy (e.g., hydrophobic nature) of many filters. U.S. Pat. Nos. 4,906,379 and 5,000,848, which are assigned to Membrex, Inc., assignee of the present application, disclose chemical modification to increase the surface free energy (e.g., hydrophilicity) of filter surfaces. (All of the documents identified, discussed, or otherwise referenced in this application are incorporated herein in their entirety for all purposes.) In general, however, relatively little attention has been given to modifying surface chemistry to reduce filter fouling.
In contrast to the chemical nature of most fouling problems, the formation of a boundary layer near the surface of the filter is physical in nature, arising from an imbalance in the mass transfer of feed fluid components towards the filter surface as compared to the back-transfer from the boundary layer to the bulk feed fluid. Some form of force (for example, mechanical, electro-kinetic) must be used to promote the desired mass transfer away from the filter surface. Unfortunately, few strategies have been developed that promote adequate back-mixing to reduce the boundary layer or prevent its formation.
The most common strategy is called "cross-flow" filtration ("CFF") or "tangential flow" filtration ("TFF"). In principle, the feed fluid is pumped across (i.e., parallel to) the outer surface of the filter at a velocity high enough to disrupt and back-mix the boundary layer. In practice, however, cross-flow has several disadvantages. For example, equipment must be designed to handle the higher flow rates that are required, and such higher flow rates generally require recirculating retentate. However, recirculation can injure certain materials that may be present in the fluid (e.g., cells, proteins) and make them unsuitable for further use (e.g., testing).
A different approach to eliminating the stagnant boundary layer involves decoupling the feed flow rate from the applied pressure. With this approach, a structural element of the filtration device, rather than the feed fluid, is moved to effect back-mixing and reduction of the boundary layer. The moving body may be the filter itself or a body located near the filter element.
Some of the rare moving-body devices that have enhanced filtration without energy inefficient turbulence are exemplified in U.S. Pat. No. 4,790,942, U.S. Pat. No. 4,867,878, U.S. Pat. No. 4,876,013, U.S. Pat. No. 4,911,847, and U.S. Pat. No. 5,000,848 (assigned to Membrex, Inc.). These patents each disclose the use of filtration apparatus comprising outer and inner cylindrical bodies defining an annular gap for receiving a feed fluid. The surface of at least one of the bodies defining the gap is the surface of a filter, and one or both of the bodies may be rotated. Induced rotational flow between these cylinders is an example of unstable fluid stratification caused by centrifugal forces. The onset of this instability can be expressed with the aid of a characteristic number known as the Taylor number. Above a certain value of the Taylor number, a vortical flow profile comprising so-called Taylor vortices appears. This type of secondary flow causes highly efficient non-turbulent shear at the filter surface(s) that reduces the stagnant boundary layer thickness and, thus, increases the permeate flux.
In contrast to classic cross-flow filtration, the devices of those patents allow the shear rate near the filtration surface and the transmembrane pressure to be independently controlled. Furthermore, because those two operating parameters are independent and high feed rates are not required to improve the permeate flux, the feed rate can be adjusted to avoid non-uniform transmembrane pressure distributions. Accordingly, mechanically agitated systems of this type enable precise control over the separation.
Rotary disc filtration devices also allow shear rate near the filtration surface and transmembrane pressure to be independently controlled. In such devices feed fluid is placed between the disc and oppositely disposed filtration surface that define the fluid filtration gap and one or both of the disc and filtration surface are rotated. See, e.g., U.S. Pat. Nos. 5,143,630, 5,254,250, and 5,707,517 (all assigned to Membrex, Inc.).
Despite the substantial work that has been done, the need remains for rotary separation devices (and processes using them) that have one or more of the following features and advantages: a relatively simple design; means for creating sufficient shear at the filter surface to prevent or reduce blinding or fouling of the filter; the decoupling of the fluid movement in the fluid filtration gap that creates the desired shear from the fluid movement in the main body (or reservoir) of feed fluid thereby, among other things, to allow flotation of less dense material in the feed fluid and settling of more dense material in the feed fluid; the ability to efficaciously process the feed fluid until only a small volume of feed fluid remains and to keep the filter wet even as the amount of fluid decreases to only a very small volume; the ability to pressurize the fluid in the fluid filtration gap, which may aid the filtration process, without the need for high pressure vessels for holding the main body (reservoir) of feed fluid; the ability to control the amount of feed fluid entering the fluid filtration gap per unit time independent of the shear caused, e.g., by Taylor vortices and independent of the permeate flow; the possibility of eliminating the use a feed pump, the elimination of which can be most advantageous if the feed fluid contains solids or other materials that would making pumping such fluid less desirable and/or difficult; and the ability to process a reservoir of feed fluid that may not be in a conventional vessel (such as fluid in a lake or fermentation vessel). Other technical problems that may be solved by the present invention will be apparent to one skilled in the art from this disclosure.